An exhaust pipe connected to a vehicle engine becomes significantly hot in driving operation because combustion gases (exhaust gases) flow inside thereof. In a high-load and high-revolution area of the engine, fuel is increased so as to avoid a rise in temperature of exhaust gases. In such a case, however, a problem is occurred that fuel efficiency is lowered and concentration of exhaust gases is raised, so that a discharge amount of contaminants is increased.
Further, when the temperature of the exhaust pipe is raised by a flow of exhaust gases, it causes heat degradation of the exhaust pipe.
Inside an exhaust pipe, a catalyst is provided for converting exhaust gases discharged from a vehicle engine. For example, a three-way catalyst can convert contaminants such as nitrogen carbide (HC), carbon monoxide (CO), and nitrogen oxide (NOx) which are contained in exhaust gases.
In order to convert these contaminants by a three-way catalyst more efficiently, it is necessary to maintain the three-way catalyst at a predetermined activation temperature.
However, in a high-speed operation of a vehicle engine, exhaust gases become very hot and there may be a case where the temperature of a three-way catalyst becomes out of the effective range for conversion of exhaust gases and fails to convert contaminants properly. Moreover, there may be a case where the three-way catalyst is thermally deteriorated due to high-temperature exhaust gases.
Accordingly, it is demanded that an exhaust pipe connected to a vehicle engine be capable of releasing the heat of exhaust gases flowing inside of the exhaust pipe in the high-speed operation of the vehicle engine.
For example, Patent Document 1 discloses an exhaust pipe of double pipe structure provided with a movable heat-transfer member between an inner pipe and an outer pipe of the double pipe. This exhaust pipe prevents exhaust gases from becoming very hot in high-speed operation of a vehicle engine, thereby satisfying the above demand for the exhaust pipe.    Patent Document 1: JP-A 2005-194962